Metal-enhanced fluorescence local field enhancement

Although it has been difficult to separate the effects of excitation and emission enhancement, both of these effects should be extremely sensitive functions of the shape of the metal particle, the orientation of the fluorophore, and the distance between the fluorophore and the metal, because the local-field effects depend strongly on these parameters. Many groups have studied variations in fluorescence intensity as a function of the distance between a layer of fluorophores and a number of nanostructured metal surfaces, adsorbed colloidal particles or suspended colloidal particles. Single-molecule experiments have even provided strong evidence for the existence of a local maximum in the fluorescence intensity versus distance curve. ... 

While the linear absorption and nonlinear optical properties of certain dendrimer nanocomposites have evolved substantially and show strong potential for future applications, the physical processes governing the emission properties in these systems is a subject of recent high interest. It is still not completely understood how emission in metal nanocomposites originates and how this relates to their (CW) optical spectra. As stated above, the emission properties in bulk metals are very weak. However, there are some processes associated with a small particle size (such as local field enhancement [108], surface effects [29], quantum confinement [109]) which could lead in general to the enhancement of the fluorescence efficiency as compared to bulk metal and make the fluorescence signal well detectable [110, 111]. 

Fluorescence-based measurements are already very sensitive and widely used in bio-medical analysis. However, the metallic nanostructures provide further improvement on the sensitivity and limit of detections through the enhancement of the local field. Therefore, a large number of researchers are dedicated to developing substrates for SEFS [46-52]. The effect of the geometrical parameter of the nanostructure on the efficiency of the SEFS is well illustrated in Fig. 9. In this case, the SEFS enhancement factor (SEFS enhancement factor) is plotted against the periodicity of the arrays of nanoholes in gold films. The experiments were realized by spin-coating the arrays of nanoholes with a polystyrene film doped with the oxazine 720 [48]. 

To understand the importance of spectral overlap to metal-enhanced fluorescence, it is useful to review the basics of metal-enhanced fluorescence. Metal nanostructures can alter the apparent fluorescence from nearby fluorophores in two ways. First, metal nanoparticles can enhance the excitation rate of the nearby fluorophore, as the excitation rate is proportional to the electric field intensity that is increased by the local-field enhancement. Fluorophores in such "hot spots" absorb more light than in the absence of the metal nanoparticle. Second, metal nanoparticles can alter the radiative decay rate and nonradiative decay rate of the nearby fluorophore, thus changing both quantum yield and the lifetime of the emitting species. We can summarize the various effects of a nanoparticle on the apparent fluorescence intensity, Y p, of a nearby fluorophore as ... 

Fluorescence and Enhanced Local Field of Metallic Nanoparticles... 

As shown in Fig. 20, Lukomska et al. could realize a ca. 100-times amplified fluorescence signal with Cy5 by using a Ti-Sapphire laser and NIR excitation [125]. The authors have found that the average fluorescence lifetimes of Cy5 on the SIF substrates are considerably reduced compared with the dye on a neat quartz support. In addition, they could show that for one- and two-photon excitation were rather similar, = 0.108 and 0.117 ns, respectively, indicating that the intensity enhancement is mainly due to a stronger local field near the metal surface. 

Within the dipolar model, the enhancement of the local excitation rate varies as (ala+df, where d is the distance from a metal sphere of radius a to the molecule. When both the excitation and the fluorescence are enhanced by the nanostructure, the total enhancement varies approximately as (a/a+d) It can also be seen that larger metal particles should provide fields that extend further out. For simplicity, let s consider an incident field ,(r,ffib) in vacuum, where in=l> then... 

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